This invention relates to a rock drill and more particularly to a down the hole drill incorporating reverse circulation.
Reverse circulation down the hole drills commonly are used to obtain rock and other mineral samples. This method of drilling allows the rock samples to move up through the centre of the drill hammer and then up the centre of the series of dual tube drill rods attached to the hammer, and is preferred to the conventional down-the-hole drilling method where the cuttings are brought to the surface between the drill rod and the wall of the bore hole. Reverse circulation hammers have been in use for a number of years now but are generally expensive and complicated in design and not without their functional problems. Most of these reverse circulation drills have a drill bit, secured to a wear sleeve, with a sample tube running up the centre of the wear sleeve, and a piston forming the hammer which moves about the sample tube. In addition to the above, air passages are generally created by the inclusion of an inner sleeve between the sample tube and the piston, or between the piston and the wear sleeve, or by porting in the wall of the wear sleeve or the piston has openings formed through the wall into the central passage. To accommodate these additions, either the bore of the piston is increased, or the outside diameter of the piston is reduced, both modifications resulting in a lighter piston having less surface area for the compressed air to act on and thus striking the drill bit with reduced force. Ports through the piston wall exacerbate this disadvantage. To increase the weight of the piston it is often lengthened which then results in increased hammer length and consequently increased length of other components of the drill.
It is the object of this invention to provide a reverse circulation drill which at least partially alleviates some of the above mentioned problems.
According to this invention there is provided a reverse circulation rock drill comprising a backhead, a cylinder extending from the backhead, a piston forming a hammer axially movable within the cylinder, and located to strike a drill but mounted to project from the cylinder at the opposite end to the backhead, a sample tube extending from the drill bit into the backhead, the outer wall of the sample tube profiled partway along its length to form with the interior of the piston an air flow path to lift the piston, the piston having its external wall profiled to form with the cylinder a second air flow path to supply air power to the piston, both air flow paths exhausting beyond the drill bit and operatively out to atmosphere through the sample tube.
Further features of this invention provide for the cylinder to be integral with the drill bit and movable along the backhead, or for the cylinder to be secured to the backhead with the drill bit constrained for limited axial movement within the cylinder.
The invention also provides a check valve in the backhead comprising an O-ring seal located on a surface tapering outwardly from the valve opening.
Still further features provide for passages to be provided through the wall of the sample tube inclined away from the drill bit end and for these passages to be provided adjacent the drill bit end or in the backhead or both.